In a multi-subscriber, single-carrier communication system employing time division multiplexing (TDM), a channel may be shared by multiple subscribers in a time division fashion; that is, the channel may be used by one subscriber at one time and by another subscriber at another time.
The time usage allocation for TDM may be fixed or variable. For fixed usage, each subscriber utilizes the channel at a fixed, pre-scheduled time slot, typically seen in time division multiple access (TDMA) systems.
Therefore, no frequent scheduling/rescheduling is needed. However, fixed channel usage may lead to resource waste, especially in bursty packet data transmission, because the subscriber may have nothing to transmit during its assigned time slot. On the other hand, with variable time usage, a channel may be used by one subscriber for a variable period of time (e.g., depending on its data load) and then used by another subscriber. With careful scheduling, variable time usage achieves statistical multiplexing gain and is typically more efficient. For examples of TDM with variable time usage, see Bender, Black, Grob, Padovani, Sindhushayana, and Viterbi, “CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users,” IEEE Communications Magazine, Vol. 38, No. 7, pp. 70-77, July, 2000.
In a multi-carrier communication system, each subscriber may be allocated multiple carriers and can use the multiple carriers simultaneously.
For a specific subscriber, the transmission rate/reliability (performance) of different carriers may be different. Furthermore, for a specific carrier, the transmission rate/reliability for different subscribers may be different. One example of such is orthogonal frequency division multiple access (OFDMA). In OFDMA, multiple subscribers simultaneously use different frequency subcarriers in a manner similar to frequency division multiple access (FDMA). For more information, see Sari and Karam, ‘Orthogonal Frequency-Division Multiple Access and its Application to CATV Networks/’ European Transactions on Telecommunications, Vol. 9 (6), pp. 507-516, November/December 1998 and Nogueroles, Bossert, Donder, and Zyablov, ‘Improved Performance of a Random OFDMA Mobile Communication System,’ Proceedings of IEEE VTC'98, pp. 2502 -2506.
Due to the unique properties of multi-carrier systems described above, the TDM scheduling algorithms designed for single-carrier systems may not directly apply. This is at least partially because, in a multi-carrier system, unlike in a single-carrier system, the operation of each carrier is to some extent dependent on each other carrier; each carrier impacts each of the others. In a single-carrier system, unlike in a multi-carrier system, there are no complications regarding orderings of packets among carriers. In a multi-carrier system, varying delays associated with multiple carriers introduce complexities unfathomed in a single-carrier system. In a single-carrier system, unlike a multi-carrier system, there needs not be any consideration given toward allocating data packets among more than one carrier. The algorithms needed to potentially optimize overall throughput in a multi-carrier system are inherently different from algorithms used in a single-carrier system.